Method of producing diffusion alloy cases



April 30, 1935. J. w. HARSCH 1,999,757

METHOD OF PRODUCING DIFFUSION ALLOY CASES Filed Jan. 50, 1929 2Sheets-Sheet 1 4-3 Jo l April 30,1935. J. w. HARSCH 1,999,757

METHOD OF PRQDUCING DIFFUSION ALLOY CASES Filed Jan. 30, 1929 '2Sheets-Sheet 2 Patented Apr. 30, 1935 METHOD or PRODUCING DIFFUSIONALLOY CASES John W. Hal-sch, Ambler, Pa., assignor to Leeds &

Northrup Company, Philadelphia, Pa., a corporation of- PennsylvaniaApplication January 30,

16 Claims.

My invention relates to a method of treating one or more metal objects,including machine parts, to produce a case thereon; and moreparticularly to a method of case-formation involving concurrent heattransfer and chemical treatment by the forcible convection of a fluid,as a gas.

In accordance with my invention, there is formed on oneor more metalobjects an improved diffusion alloy case, as a nitrided or carburizedcase, having uniform surfacecharacteristics and uniform gradient ofcharacteristics in the subsurface, by substantially uniformlyconcentrating throughout the surface of the metal the reactive orcase-forming component of a heated gaseous fluid medium by forciblystirring it in contact with the heated metal, and concurrently by theforcible stirring of the fluid medium maintaining the metal throughoutits said surface at substantially uniform case-forming reactiontemperature. Besides producing the aforesaid characteristics, which indiffusion alloy cases are of great commercial and practical importance,my method is characterized by increase in the rate of the caseformingreaction, by material reduction in the duration and cost of treatment,and by regularly and repeatedly procuring the same uniform results.

Further and more particularly in accordance with my invention, areactive gas or vapor, as ammonia for nitriding, is circulated into heattransfer relation with a source of heat where it is dissociated to agreater or less extent and utilized as a heat vehicle; and moreparticularly the aforesaid gas or vapor in its wholly or partiallydissociated state is subsequently passed into direct contact with themetal for case-forming reaction therewith concurrent with absorption ofheat by said metal from the reacting gas or vapor.

My invention resides in the methods of a caseformation hereinafterdescribed and claimed.

In case-forming processes involving both application of heat and achemical agent for effecting desired changes, either chemical orphysical, or

I, both, in the properties of the metal, it has pre- 1929, Serial No.336,0 5 (01. 148-16) phere be non-uniform, since two independentlycontrolled factors are involved.

My invention comprises heating the metal and chemically treating thesame by a single fluid medium or heat vehicle, as a gas, concurrentlyand in one process. The gas, which may be of whatever composition isrequired for the particular needs of the treatment, therefore serves atwo-fold purpose in that it is the principal medium or vehicle of heattransfer to the metal for effecting heating or equalization of thereaction temperature thereof, and at the same time reacts chemicallywith the metal itself to produce desired properties. v

In practice, the chemically active gas is introduced under pressure intoa heating chamber, where it is passed into heat-transfer relation with asource of heat, whereupon it is utilized as a heat vehicle andsubsequently passed into heattransfer relation with metal to effectuniform chemical treatment thereof, concurrent with uniform heating ofthe metal to reaction temperature by absorption of heat from the gas.Accordingly, it is possible to insure uniformity of both the heat andchemical treatments, since the gaseous heat vehicle in flowing aroundthe metal must necessarily chemically react therewith at the same timeas the gas gives up part of its heat to the metal. Uniformity oftreatment due to the fact that the chemically active gas is also theheating gas, is furthermore insured by the fact that the temperature ofthe metal is a determining factor in the reaction between the gas andmetal. In other words, assuming that the metal is heated independentlyof the treating gas, the situation might arise wherein differentportions of the batch of metal under treatment are at differenttemperatures, and further assuming uniform application of the treatinggas, it is apparent that, notwithstanding the uniform application, thechemical treatment'of the metal will be non-uniform as well as the heattreatment thereof. The method of obtaining uniformity of heat treatmentby circulation of a heated gas into heat-transfer relation with a metalhas been practiced heretofore, and in itself does not constitute a partof my invention. By utilizing a chemically active gas,

. however, in lieu of the usual heating medium, uniform chemicaltreatment is assured due to the even temperature of the batch of metal,and furthermore due .to the fact that the heating gas is caused to flowevenly under substantially constant pressure throughout all parts of thebatch.

For an illustration of apparatus capable of 55 carrying out my method,which is claimed in my divisional application Ser. No. 511,694,reference is had to the accompanying drawings, in which:

Fig. 1 is an elevational view, partly in section, of a furnace.

Fig. 2 is a plan sectional view of the apparatus illustrated in Fig. 1taken along the line 22.

Fig. 3 is a detailed view of apparatus associated with the furnace.

Referring to Fig. 1, there is illustrated a furnace or heating structurecomprising an outer cylindrical shell I having its lower portion closedby a bottom plate 2 suitably secured to the lower edge of shell l, as bybolts 3 to flange 4. To the bottom plate 2 is secured thefurnace-supporting structure 5. The upper edge of shell I is united to asimilar shell 6 having an inwardly turned flange 1 whose inner edge issecured and sealed, as by weld 8, to the upper edge of a shell 9,disposed concentrically within, and spaced from shell I by insulatingmaterial ID of suitable characteristics. Shell 9, which comprises theinner lining of the furnace, is open at its upper end, and is closed atits lower end by members and I2, members 9, H and I2 being secured toeach other by the hermetically sealed welded joints l3. Member l2,comprising the bottom portion of the furnace chamber, is secured andspaced with respect to plate 2 by members M which are also welded tomember 9 for purposes of sealing. Upright supports 5, comprising angularmembers, are mounted upon the bottom of the furnace chamber by means ofstuds Ma, and support a plurality of heating elements, as electricalresistors l6, by means of annular supporting bands Ilia: and insulatorsIGy. Mounted also upon supports I 5 is a cylindrical shell-like memberopen at its top and bottompand Within which a work container I8 isconcentrically disposed. The work container l8 comprises a cylindricalshell-like wall having an open grille work or spider l9 secured to thelower edge thereof, and an annular supporting flange 20 secured to theupper edge thereof adapted to seat upon the upper edge of cylinder l1,and thereby closing the intervening space from circulation of gases.Supporting flange 2|] is provided with eyes or handles 2| forfacilitating withdrawal of the work container from the furnace.

Suspended beneath the furnace by means of structure 22 and through-bolts22a is a motor M having the axis of its rotor shaft disposed in avertical plane. Rotor shaft 23 extends through the bottom plates 2 andI2, and is substantially sealed with respect to the furnace walls bymeans of a packing gland 24 and a sleeve or collar 25 forming a part ofsaid gland and welded to the bottom member l2 at 26. Shaft 23 terminatesbeyond a conical flow-deflecting member 21, and has secured to the endthereof a fan or impeller 28.

Resistors l6 are connected to an external source of electro-motiveforceby means of lead members Mia and ||ib suitably mounted andprotected within conduit or housing member |6c extending through thewall of the furnace. The inner end of conduit IE0 is welded at |6d tothe co-acting edge of shell 9, and is provided with an electricalinsulating bushing 29. Conduit I60 has mounted at its outer end a member30 closed by a plug 3| at its lower end and supporting a sleeve-likemember 32 for supporting the lead conductor I612. The conducting members16a and "lb are suitably spaced and insulated with respect to theirsupporting members, as by an electrical insulating bushing 33, asbestospacking 34 and an insulating cement 35 which also serves to insuresealing of the conduits from the exterior against gas leakage. Anelectrical insulating bushing 36 and a lead terminal 31 are disposed atthe end of sleeve 32. Although but one outlet supply conduit has beendescribed and illustrated, it will be noted, referring to Fig. 2, that aproper number of them are supported by and extend through the furnacewall, for supplying current to the resistors, depending upon the powercharacteristics desired.

Cover structure for the furnace comprises a cylindrical shell-likemember 38 of somewhat larger diameter than member 6, having the dishedplate-like members 39 and 40 suitably secured thereto, member 39 beingwelded at its outer periphery to member 38 for purposes of hermeticallysealing the junction of these members. Members 39 and 40 are spaced fromeach other forming an enclosure for insulating mate-- rial of suitablecharacteristics, and have mounted therein a thermo-responsive device 4|comprising a supporting tube 4|a welded on its outer surface at 4|b tomember 39 and sealed by suitable cement or equivalent at Me. Thethermoresponsive device 4|, as a thermo-couple, and a fluid conduit 42,extend through the cover structure to communicate with the furnaceinterior. Conduit 42, welded at 42b to member 33, is connected to aflexible tube or conduit 43 having interposed therein a measuringdevice, as-a flow meter 44 and a flow controlling device, as a needlevalve 45 for controlling the flow from a supply conduit 46.

The lower portion of shell 38 comprises a sleeve-like structuresurrounding the outer wall of the furnace and co-acting with a liquidseal, as an oil seal 41 for hermetically sealing the furnace chamberwith respect to the atmosphere. The seal 41 comprises an annular wallstructure 48 having its lower portion sealed at 43a, as by welding, tothe outer side of member 6, and forming therewith an annular receptaclefor contain-.

ing a sealing liquid such as oil, for example. As illustrated in Fig. 1,the lower portion of shell 38 extends into the oil to a suitable depth,thereby efi'ectively sealing the interior of the furnace from theatmosphere. Suitable heat' resisting and resilient material, such asasbestos rope 49, is disposed along the upper edge of the furnace wallto provide a seat for the above described cover structure. The coverstructure, including the thermo-responsive device 4| and the conduit 42,may be lifted as a unit from the furnace by hoisting means connected tothe lifting hook or eye 50.

The furnace is provided with an outlet or exhaust conduit 5|, welded at5|a to the furnace lining, having seated at its outer end a gasketedclosure member or plug 52 adapted to be unseated to open the outer endof the conduit by a member 53, pivoted at 54, and connected to theclosure member through a pivoted connection 55. The closure member 52may be maintained in snug and sealing engagement with the end of conduit5| by means of the screw clamp 56 pivotally mounted at 51.

Referring more particularly to Fig. 3, conduit 5| communicates with aconduit 58 terminating in a liquid seal-comprising a container 59, asealing liquid 60 and an outlet or exhaust conduit 6|. During closure ofmember 52, exhaust from the furnace chamber may take place throughconduit 5|, conduit 58 and the liquid seal above described, provided ofcourse that sufficient pressure exists in the furnace chamber.

The material of the exposed parts of the in'- terior furnace structure,such as the lining 9 and shell H, which serves to shield the work withincontainer 18 for direct radiant heat. from the resistors, and otherstructure subject to contact by the particular gaseous medium utilizedin the furnace chamber, comprises a metal or alloy thereof resistant tocorrosive action of the heat and gaseous medium, such as for example,high chromium iron, or chromium-nickel iron alloys when ammonia isutilized as the gaseous medium. The electrical resistors l6 which arealso exposed to the action of the furnace gas or gases are also composedof a corrosive-resisting material, and by way of example a nickel-chromealloy is used in an atmosphere of ammonia.

The liquid seal at the exterior of the furnace is substantially at roomtemperature and is not appreciably affected by the high temperaturesexisting within the furnace, thereby preventing rapid deterioration orevaporation of the sealing fluid.

' The operation of the system is as follows;

Assuming the furnace to be in condition for recharging, the coverstructure is removed by suitable lifting or hoisting mechanism securedto member 50, after which work container I8 is lifted from the furnaceby means of eyes 2|. A work container filled with material to be treatedis subsequently lowered into the furnace to the position shown in Fig.1, after which the cover structure is lowered into its sealing position.In order that the furnace chamber may be entirely free from any gasother than that used in the treating process, the furnace is flushed foran appreciable length of time by the treating gas. To this end, needlevalve 45 is opened, permitting flow of the treating gas under pressurethrough conduits 46, 43 and 42 to the interior of the furnace. Motor Mis thereupon energized to rotate fan 28, which circulates the treatinggas, depending on the direction of rotation of the fan, between theinner work containing chamber and the outer annular heating chambercontaining resistors I6. In this manner, the treating gas is caused tocirculate throughout all parts of the furnace interior to effectivelyflush out all gases such as air or other oxygen-containing gases, and toeffect discharge of the same through exhaust conduit 5| and the liquidseal 60. During the above described flushing process, resistors l6 aredeenergized and generate no heat, so that the new batch of material tobe treated remains comparatively cold and is not acted upon to anyappreciable extent by the flushing treating gas. Since the treating gasis generally introduced into the interior of the furnace atcomparatively low pressure, the pressure therein during normal operationwill be somewhat above atmospheric, so that there is a continuous flowof gas through the exhaust conduit and seal, thereby insuring acontinuous supply of fresh treating gas.

After the flushing process has beencompleted, resistors iii areenergized from a source of power (not shown), and the furnace interioraccordingly increases in temperature. Assuming now that fan 28 rotatesin such direction that the furnace gas is caused to flow upwardlythrough the open grille or spider IQ of the work container I8, throughthe batch of work therein, the gas will return to fan 28 by way of theannular heating chamber, and flow around and contact with the highlyheated resistors during the downward a furnace effects uniformity ofheating of the work therein, and is fully described and claimed in myPatent 1,518,027, March 23, 1926.

After the furnace interior has been brought up to the desiredtemperature, it may be maintained substantially constant by well-knowncontrol means associated with the thermo-responsive'device 4|. Duringthe treatment, the treating gas is caused to flow continuously throughconduit 42 to replenish the gases which are exhausted during thetreatment, and to force out of the furnace through the exhaust seal suchexhausted gases. In general, the temperature of the work itself is adetermining factor in governing the rate and/ or extent of reaction ofthe treating gas with the work. By utilizing the treating gas itself asthe principal heat vehicle between the source and the work, thecylindrical shield I! together with container I8, both of which areun-perforated. effectively preventing appreciable transmission ofradiant heat to the work, it is possible to effect gradual and uniformheating of the work while at the same time subjecting it to the even anduniform flow of the treating gas or gases, whereby the gas and the metalor material to be treated are concurrently in heat-transfer andchemicallyreacting relations.

Although no definite rate of circulation of the combined heat vehicleand treating gas is contemplated, the fan or impeller 28 should rotateat such speed that the circulating medium effectively removes or wipesoff stagnant films on the surfaces of the work under treatment, in orderthat the rate of treatment may be materially increased, instead of beingreduced by the existence of stagnant films which tend to prevent orretard the desired reaction between the treating gas and the work.

When the work within the furnace has been subjected to a predeterminedduration and extent of treatment, the electrical resistors aredeepergized and the fan motor shut off.

Before the removal of the furnace cover, however, the exhaust conduit 5|is opened to atmosphere so that removal of the cover structure will notcreate a vacuum within the furnace interior and so cause the liquidwithin seals 41 and 69 to be drawn into the furnace. To this end,scaling plug 52 is released by unloosening clampingv nut 56, after whichlever 53 is rotated in a counter-clockwise direction to move plug 52 outof sealing engagement with the end of conduit 5!. Accordingly, theinterior of the furnace is now directly in communication with atmosphereand hoisting of the cover structure cannot therefore create a vacuumwithin the furnace to break and disrupt the aforesaid liquid seals. Itis essential that the sealing plug 52 be open only while the coverstructure is being lifted from the furnace to permit removal of thework, since replacement of the cover would only tend to force excess airthrough the exhaust seal.

An example of the use of a chemically active gas for effectingconcurrent heat and chemical treatment of a metal lies in the use ofammonia, part of which is dissociated into nitrogen and hydrogen for thenitriding of steel. As is well known in the art, a nitrided steel orsuitable alloy thereof has valuable wear-resisting characteristies, thenitrided material having a very hard outer surface or shell highlyresistant to wear or abrasion. An example of a practical use to which anitrided steel may be put, lies in its application to shafts orequivalent members incorporated in high speed machinery, such as inautomotive engines.

Before the nitriding treatment is actually started, ammonia gas or vaporis caused to circulate, in the manner previously described, by the fanfor an appreciable length of time, generally about an hour, through thetreating chamber and other parts of the furnace to the exterior throughthe exhaust seal, in order that air or any other undesirable gas, as anoxygen-containing gas for example, shall be completely forced out of thefurnace. During this gas flushing period, the batch of metal to betreated is not acted upon by the ammonia, since the electrical resistorsare deenergized. After the flushing period has been completed, thecircuit through the resistors is closed, and the furnace interioraccordingly increases in temperature. As the fresh ammonia gas isintroduced under pressure into the upper portion of the furnace, it iscaused to flow downwardly either through the treating chamber or theheating chamber, depending on the direction of rotation of the fan, andto circulate between the source of heat and metal. As the gas flows pastthe highly heated electrical resistors, it is dissociated to a greateror less extent by the heat absorbed from the resistors, and accordinglyis adapted to react with the metal under treatment when the same. hasbeen heated sufficiently by the heat-carrying gas to maintain thedesired chemical action. As the ammonia gas, which comprises the heatvehicle, brings the metal up to the desired temperature, furtherdissociation of the ammonia occurs during its contact with the heatedmetal. However, since the source of heat, namely the resistors, is at ahigher temperature than that of the metal under treatment, it will beapparent that dissociation of the ammonia occurs to a greater extentwhen it is passed into heat-transfer relation with the source, and so isin a chemically active state before coming into contact with the metal.

As an individual charge of ammonia would become exhausted within acomparatively short period of treatment, a continuous supply,controllable by the furnace operator, is admitted to the furnace as theweakened or exhausted gas is discharged from the furnace through theyielding exhaust seal to atmosphere, or to gas recovery apparatus.

My invention comprises numerous important commercial advantages, not theleast important of which is the rapidity of treatment attained by theaforesaid method. By way of example, the actual time of an individualnitriding treatment has been reduced by my invention to approximatelyone-third of the time formerly required by other methods.

A carbonaceous gas may also be used in the above described manner as theprincipal me dium of heat transfer for the purpose of gas carburizingthe metal to give also a greater degree of hardness thereto. Thecarbonaceous gas may obviously vary somewhat in composition, dependingupon the degree of hardening required.

The forcible stirring of the gaseous atmosphere, having a case-formingreactive component, while in contact with the work ensures throughoutthe exposed surface thereof both that the concentration of the reactivecomponent of the gas is maintained substantially uniform, and that asubstantially uniform reaction temperature is maintained. As a result,the case produced upon the one or more objects under treatment is ofsubstantially uniform surface concentration and of substantially uniformsubsurface characteristics; more specifically in nitriding, the nitridcase is of substantially uniform surface hardness and the hardnessgradient from the surface is substantially uniform throughout the work;and in carburizing, the concentration of carbon and the reactiontemperature at and near the surface are substantially uniform throughoutthe surface of the work, and the case eventually resulting from thoseconditions is of substantially uniform surface hardness and ofsubstantially uniform hardness gradient inwardly from the surface.

With all conditions maintained the same, excepting only omission of theforcible stirring of the fluid medium, the reaction products formedfrequently differ from those formed when the fluid medium is forciblystirred, and whatever the nature of the reaction products theiroccurrence in the surface and subsurface is of a character which failsto effect a diffusion alloy case of uniform surface characteristics anduniform gradient of subsurface characteristics.

It is also within the scope of my invention to utilize a gaseous mixturecomprising two or more gases, or a vaporized liquid, as the principalmedium of heat transfer to effect concurrently a plurality oftreatments. It will therefore be understood that the term gas as used inthe above specification and appended claims is not intended to belimited to a single gas, or vaporized liquid, but is intended to includea gaseous mixture of two or more gases in whatever proportions may befound desirable.

It will furthermore be understood that my method of utilizing a gas bothas the principal medium of heat transfer to or temperature equalizationof a metal, and to effect formation of a case thereon is not limited tothe specific type of furnace herein disclosed, but is applicable toother types of furnaces wherein heating medium is forcibly circulatedeither in continuous or reverse directions, and wherein heat istransmitted to the heat-receiving substance by both radiation andconvection. Furthermore, the heat may be developed by any suitablemethod, as by gas firing, electrical resistors or other means. Ingeneral, it is essential that the furnace interior be hermeticallysealed with respect to the at mosphere in order that the treating gas,which may be irritating or obnoxious to the attendant, be entirelyconfined within the furnace, and that air may also be prevented fromleaking into the furnace and producing undesirable results.

I am aware it has been suggested for carburizing to burn fuel to producethe furnace heat and by fan to pass the products of combustion over thework in contact therewith; such method is ineffective for the purposesof my invention because of the presence in the gas in contact with thework of a substantial proportion of carbon dioxide, resulting from thecombustion, which is a decarburizing agent. I disclaim herefrom anymethod of carburizing in which the gaseous atmosphere in contact withthe work to substantial extent comprises products of combustion.

As distinguished from such results as the effects of oxidizing orreducing gases, the case referred to herein and in the appended claimsis a true case, a diffusion alloy case, comprising the metal, on whichthe case is formed, alloyed at and beneath the surface with compounds ofthe metal formed by reaction therewith of the case-forming reagent; andI disclaim hercfrom and from the appended claims all methods exceptthose producing 01' forming diffusion alloy cases.

What I claim is:

1. The method of producing upon one or more definitely formed metalobjects a diffusion alloy case of substantially uniform surfaceconcentration and of substantially uniform sub-surface characteristics,which comprises forcibly stirring a hot gas, having a component ofcharacter to react with the metal to form the diffusion alloy case, incontact with the metal to effect substantially uniform concentration ofsaid reactive component throughout the surface of the metal whileutilizing said gas to maintain said surface throughout at substantiallyuniform reaction temperature.

2. The method of imparting to one or more definitely formed metalobjects a nitrid case substantially uniform in surface hardness and ofsubstantially uniform sub-surface characteristics,

' which comprises forcibly stirring at hot gas, containing a nitridingcomponent, in contact with the metal to effect substantially uniformconcentration of said nitriding component throughout the surface of themetal while utilizing said gas to maintain said surface throughout atsubstan- ,tially uniform nitriding temperature.

3. The method of imparting to one or more definitely formed metalobjects a diffusion alloy case substantially uniform in surface carbonconcentration and of substantially uniform sub-surface characteristics,which comprises forcibly stirring a hot gas, containing a carburizingcomponent. in contact with the metal to effect substantially uniformconcentration of said component throughout the surface of the metalwhile utilizing said gas to maintain said surface throughout atsubstantially uniform carburizing temperature.

4. The method of nitride hardening a metal article which comprisesheating the article to a nitriding temperature by circulating a heatedgas about it in a nitriding zone closed to the atmosphere and thereaftercausing ammonia to pass in cyclic circulation between an activating zoneand said nitriding zone, said ammonia being subjected in said activatingzone to the dissociating action of a catalyzer and heated sufficientlyto maintain a nitriding temperature in said nitriding zone.

5. The method of producing a diffusion alloy case upon one or morepieces of metal which comprises effecting generation of heatformaintaining the metal at case-forming reaction temperature, into onezone introducing a case-forming reagent produced independently of saidgeneration of heat and effecting partial dissociation thereof, passingthe partially dissociated agent into a second zone containing the metal,and forcibly stirring the treating agent in contact with the metal, toeffect further dissociation of the treating agent, to remove film fromthe surface of the metal, to ensure uniform reaction between the metaland the dissociation product, and substantially to enhance the speed ofreaction between the metal and the dissociation product.

6. The method of producing a diffusion alloy case upon one or morepieces of metal by carburization thereof, which comprises effectinggeneration of heat passing into a zone a treating agent producedindependently of said generation of heat, having a carburizingeomponent,-in said zone effecting partial dissociation thereof, passingthe partially dissociated agent into a second zone containing the metal,and forcibly stirring the treating agent in contact with the metal, toeffect further dissociation of the treating agent to facilitatecarburization of the metal, and to remove film from the surface of themetal.

7. The method of producing a diffusion alloy case upon one or morepieces of metal, which comprises heating the metal to case-formingreaction temperature in .a reaction zone closed to the atmosphere,causing gas having a case-forming component to pass in repeated cycliccirculation between an activating zone and said reaction zone, said gasbeing subjected to partial disso- 'ciation in said activating zone, andforcibly stiruniformity and speed of formation of the case whichcomprises effecting generation of heat, forcibly stirring in contactwith the heated metal a gas. having a component of character to reactwith the metal to form the diffusion alloy case, to

effect substantially uniform conwntration of said reactive componentthroughout the surface of the metal and to effect convective transfer ofheat between the gas and the metal to maintain its surface atsubstantially uniform reaction temperature, and independently, of saidstirring supplying reagent, produced independently of said generation ofheat, to replenish the case-forming component of said gas.

9. The method of producing a uniform diffusion alloy ease upon metalobjects, which comprises raising the temperature of a fluid, having acomponent of character to react with the metal to form the diffusionalloy case, by convective absorption of heat generated independently ofthe supply of said fluid, and forcibly stirring the heated fluid intocontact with the metal to effect substantially uniform concentration ofsaid reactive component throughout the surface of the metal whileeffecting convective transfer of heat from the fluid to the metal tomaintain its surface throughout at substantially uniform reactiontemperature.

10. The method of forming a diffusion alloy case upon metal objects,which comprises effecting generation of heat for heating the objects tothe temperature of dissociation of a treating fluid, having a componentof character to react with the metal to form the diffusion alloy case,forcibly circulating the fluid into contact with said heated objects toeffect substantially uniform concentration of said reactive componentthroughout the surface of the metal while utilizing the fluid to effectby convection equalization of temperature of the metal to maintain itssurface throughout at substantially uniform reaction temperature, andsupplying reagent, produced independently of said generation of heat, toreplenish the caseforming component of said fluid.

11. In a system for producing a diffusion alloy case upon metal objectscomprising a furnace 'having a work chamber containing the metal, themethod which comprises forcibly stirring in said chamber a fluid, havinga component of character to react with the metal to form thediifusionalloy case, concurrently to effect substantially uniform concentrationof said reactive component throughout the surface of the metal, andinterchange of heat between the fluid and metal, without interruption ofsaid stirring of the fluid adding fresh fluid to and removing fluid fromsaid chamber in quantities small as compared with the quantity of fluidin the furnace, and effecting generation of the heat, for maintainingthe reaction temperature, independently of the supply of said fluid.

12. The method of producing on one or more pieces of metal asubstantially uniform diffusion alloy case, which comprises forciblystirring in contact with the metal a hot gaseous fluid, having acomponent of character to react with the metal to form the diffusionalloy case, to clear the surface of the metalfor facilitating thecase-forming reaction, and to effect substantially uniform and enhancedconcentration of said reactive component throughout the surface of themetal while utilizing said gaseous fluid to insure uniformity ofreaction temperature throughout the surface of the metal and effectinggeneration of the heat, for maintaining the reaction temperature,independently of the supply of said fluid.

13. The method of producing upon one or more metal objects a diffusionalloy case of substantially uniform surface concentration and ofsubstantially uniform subsurface characteristics, which compriseseffecting generation of heat,

' forcibly stirring a hot gas, having a component of character to reactwith the metal to form the diffusion alloy case, in contact with themetal to effect substantially uniform concentration of said reactivecomponent throughout the surface of the metal while utilizing said gasto maintain said surface throughout at substantially uniform reactiontemperature, and supplying material, produced independently of saidgeneration of heat, to replenish the case-forming component of said gas.

14. The method of imparting to one or more metal objects a diffusionalloy case substantially uniform in surface carbon concentration and ofsubstantially uniform subsurface characteristics, which compriseseffecting generation of heat for maintaining the work at case-formingreaction temperature, forcibly stirring a hot gas, having a carburizingcomponent produced independently of said generation of heat, in contactwith the metal to effect substantially uniform concentration of saidcomponent throughout the surface of the metal while utilizing said gasto maintain said surface throughout at substantially uniform carburizingtemperature, and supplying reagent to replenish the carburizingcomponent.

15. The method of producing upon one or more metal objects a diffusionalloy case of substantially uniform surface concentration and ofsubstantially uniform subsurface characteristics, which comprisessubjecting the metal to a gaseous medium having a component of characterto react with the metal at elevated temperature to form the diffusionalloy case, electrically generating heat to provide the reactiontemperature, substantially uniformly concentrating said reactivecomponent throughout the surface of the metal by forcibly stirring theheated medium while in contact with the heated metal, and concurrentlyby said forcible stirring of said medium maintaining the metalthroughout its said surface at substantially uniform reactiontemperature.

16. The method of producing upon one or more metal objects a carburizeddiffusion alloy case of substantially uniform surface carbonconcentration and of substantially uniform subsurface characteristics,which comprises subjecting the metal to a gaseous medium having acarburizing component of character to react with the metal at elevatedtemperature to form the diffusion alloy case, electrically generatingheat to provide the reaction temperature, substantially uniformlyconcentrating said carburizing component throughout the surface of themetal by forcibly stirring the heated medium while in contact .with theheated metal, and concurrently by said forcible stirring of said mediummaintaining the metal throughout its said surface at substantiallyuniform reaction temperature.

JOHN W. HARSCH.

